In the prior art, so-called Ziegler-Natta catalyst consisting of a titanium or vanadium compound as a primary catalyst component and an alkylaluminum compound as cocatalyst component have been generally used for preparing ethylene homopolymers or copolymers of ethylene and α-olefin. Although a Ziegler-Natta catalytic system exhibits high activity on ethylene polymerization, the catalytic system has disadvantages in that molecular weight distribution of the produced polymer is broad due to non-uniform catalyst activation point, and especially, composition distribution thereof is not uniform in the copolymers of ethylene and α-olefin.
Since a metallocene catalyst system composed of a metallocene compound of a Group 4 transition metal in the Periodic Table of Elements, such as titanium, zirconium, hafnium, or the like, and methylaluminoxane as a cocatalyst is a homogeneous catalyst having a single catalytic active site, the metallocene catalyst system may prepare a polyethylene having a narrow molecular weight distribution and a uniform composition distribution as compared to the existing Ziegler-Natta catalyst system. For example, in European Patent Application Publication Nos. 320,762 and 372,632, or Japanese Patent Laid-Open Publication No. Sho 63-092621, Hei 02-084405, or Hei 03-002347, it was reported that ethylene may be polymerized with a high activity by activating a metallocene compound such as Cp2TiCl2, Cp2ZrC12, Cp2ZrMeCl, Cp2ZrMe2, ethylene(IndH4)2ZrCl2, or the like, using methylaluminoxane as the cocatalyst, thereby making it possible to prepare a polyethylene having a molecular weight distribution (Mw/Mn) in a range of 1.5 to 2.0. However, it is difficult to obtain a polymer having a high molecular weight using the catalyst system, and particularly, in the case of applying the catalyst system to a solution polymerization method performed at a high temperature of 120° C. or more, a polymerization activity is rapidly decreased, and β-dehydrogenation reaction mainly occurs, such that the catalyst system is known to be not suitable for preparing a high-molecular weight polymer having a weight average molecular weight (Mw) of 100,000 or more.
Meanwhile, as a catalyst capable of having a high catalytic activity and preparing a high-molecular weight polymer in ethylene homopolymerization or copolymerization of ethylene and α-olefin under the solution polymerization conditions, a so-called geo-restrictive non-metallocene type catalyst (also referred to as a single-site catalyst) to which a transition metal is linked in a ring form has been reported. An example of a catalyst in which an amide group is linked to one cyclopentadiene ligand in a ring form has been suggested in European Patent Nos. 0416815 and 0420436, and an example of a catalyst in which a phenolic ligand as an electron donor compound is linked to a cyclopentadiene ligand in a ring form has been disclosed in European Patent No. 0842939. However, since during the synthesis of the geo-restrictive catalyst as described above, a yield of a process of a ring formation reaction between a ligand and a transition metal compound is low, there are many difficulties to commercially use such catalyst.
On the other hand, examples of a non-metallocene type catalyst that is not geo-restrictive have disclosed in U.S. Pat. No. 6,329,478 and International Publication No. WO 00/005238. In these patents, it may be confirmed that a single-site catalyst using at least one or more phosphine imine compounds as a ligand has a high ethylene conversion rate at the time of copolymerizing ethylene and α-olefin under a high-temperature solution polymerization condition at 120° C. or more. An example of a catalyst having a bisphenoxide ligand has been disclosed in U.S. Pat. No. 5,079,205 and an example of a catalyst having a bisphenoxide ligand of chelate type has been disclosed in U.S. Pat. No. 5,043,408. However, these catalysts have an excessively low activity, such that it is difficult to commercially use these catalysts for preparation of an ethylene homopolymer or a copolymer of ethylene and α-olefin, which is performed at a high temperature.
Use of an olefin based polymerization catalyst having an anilido ligand has been disclosed in Japanese Patent Laid-Open Publication Nos. 1996-208732 and 2002-212218, but an example thereof in a commercially significant polymerization temperature region has not been disclosed. In addition, a case in which the anilido ligand was used for polymerization as a non-metallocene type catalyst has been reported in ┌Organometallics 2002, 21, 3043 (Nomura et al.)┘. In this case, the case was confined to a methyl group which is a simple alkyl substituent.
Therefore, it is important to secure a more competitive catalyst system capable of satisfying characteristics required in a commercial catalyst on the basis of economical efficiency, that is, an excellent high-temperature activity, excellent reactivity with higher alpha-olefin, preparation capability of a high molecular weight polymer, and the like.